Usually, this is caused by:
1) fertilized eggs sitting too long in a warm place
or
2) a bit of the hen's uterus got sloughed off along the way. This doesn't happen very often, but it is normal. We just feed those eggs to the dog.
If it is happening more than twice a month, your hen might have some uterine damage. I can't think of a way to treat it though...you may just have to live with ucky eggs. Does the hen herself seem healthy, or do you not know who laid the eggs?